1. Field of the Invention
The present invention relates to paper compression rupture tests. Such tests are, for example, used for classifying papers according to their ability to form cardboard boxes for packing. Packing cardboard boxes, which are to be piled up, must have a good compression resistance.
2. Discussion of the Related Art
FIG. 1 illustrates a so-called "ring-crush" standardized compression test, which is described, for example, in "Physical and Mechanical Testing of Paper and Paperboard", volume 1, Richard E. Mark editor, 1983. A paper strip to be tested 10 is curved to form a ring. The ring is maintained in a groove of a bearing 12. The width of strip 10, its protrusion with respect to bearing 12, and the ring diameter are standardized (respectively, 12.6 mm, 6.3 mm and 47 mm). Further, strip 10 is wound substantially in one turn to form the ring.
During the test, ring 10 is crushed at constant speed by a plate parallel to bearing 12. The result of the test is the maximum force exerted during the crushing.
The results of this test are much lower than the pure compression resistance since, despite the low profile of ring 10, its walls tend to locally buckle. Thus, the results rather translate the resistance to local buckling than the compression resistance, especially for thin papers.
Further, this type of testing, due to the low height of the ring, does not enable rheological tests, that is, to acquire the curve of the compression deformation according to the force.
FIGS. 2 and 3 illustrate two compression tests which provide a more accurate evaluation of the resistance to pure compression and enable a rheology. These tests are also described in the above-mentioned document.
In FIG. 2, a paper strip 14 is grasped at each end by a respective jaw 16, 17. Between the jaws, strip 14 is guided between two plates 19, 20 which are meant to enable strip 14 to slide while preventing its buckling when jaws 16 and 17 are brought close to each other to perform the test.
A disadvantage of this system is that the setting of the spacing between plates 19 and 20 is delicate. On the one hand, they must not be too spaced apart from each other, which would cause, as is shown, undulations of the strip, due to the buckling. On the other hand, they must not be too close to each other, since strip 14 would be submitted to a strong friction during its deformation. In one case as in the other, the measures would be distorted.
For this reason, the system of FIG. 3 is preferred, where each of guide plates 19 and 20 has been replaced with a series of flexible thin plates 22 perpendicular to the plane of strip 14. Such a system enables a firm lateral holding of strip 14, while this strip can deform freely in compression between jaws 16, 17, the deformations being allowed relatively freely thanks to the flexibility of thin plates 22.
To reduce the buckling risk of strip 14 between two successive thin strips 22, the number of thin strips is multiplied, which increases the complexity and the cost of the system, without however totally suppressing the buckling risk. Further, if the number of thin plates 22 is multiplied, their resilience has a non-negligible influence upon the measure results.